1. Field of the Invention
The present invention relates to a cantilever probe used in a scanning tunneling microscope or an information processing apparatus utilizing the principle of scanning tunneling microscopy, and a method of producing the same.
The present invention also relates to a scanning tunneling microscope and an information processing apparatus which can perform recording, reproducing and erasing of information.
2. Relating Background Art
Recently, research has been conducted on the application of scanning tunneling microscopic technique to various fields, such as: (i) observation of a semiconductor or polymer on an atomic or a molecular order; (ii) fine processing (E. E. Ehrichs, Proceedings of 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, 1989. S13-3) and (iii) recording apparatus.
In a scanning tunneling microscope, a probe which measures tunnel current, is required to be small in size to (1) reduce thermal draft, (2) to increase mechanical resonance frequency and (3) to reduce sensitivity to external vibration. Therefore, probes have been made small in size using semiconductor processing techniques (C. F. Quate et al., Transducer 1989, lecture No. D3.6, June 1989).
Moreover, compact recording apparatus is also required because recording apparatus with a large amount of memory have been increasingly required to process computer or image information. The microprocessor for such apparatus has been made very compact and its ability to process information calculation has been improved by the development of a semiconductor processing technique.
For the purpose of satisfying these requirements, U.S. Pat. No. 4,829,507 proposes a recording and readout information system having atomic scale densities comprising a recording medium having a carrier and means to form a pattern of atomic particles on the surface of the carrier.
In such an apparatus, it is necessary to scan the specimen using a probe over an area of several nm to several .mu.m. A piezoelectric element is used to move the apparatus. Examples of such movement mechanisms, include the tripod type and the cylindrical type. The tripod type combines three piezoelectric elements which are perpendicular to each other along the X, Y and Z directions and a probe which is located on the intersecting point of the three elements.
A cylindrical type mechanism utilizes one end having divided electrodes provided around the peripheral surface of a cylindrical piezoelectric element. A probe is provided on the other end of the divided electrodes which is able to scan and which makes the cylinder bend corresponding to each divided electrode. Recently, attempts have been made to form a fine cantilever probe by employing micromachining techniques utilizing semiconductor processing (K. E. Peterson, IEEE Trans. on Electron Devices, vol. ED-25, No. 10, pp. 1241-1249, 1978). FIG. 31 is a perspective view of a prior art piezoelectric bimorph cantilever formed on a silicon (Si) substrate by employing a micromachining technique [T. R. Albrecht, "Microfabrication of Integrated Scanning Tunneling Microscope", Proceedings of 4th International Conference on Scanning Tunneling Microscopy/Spectroscopy, '89 S10-2]. FIG. 32 is a sectional view of the cantilever.
The cantilever is formed on a silicon substrate by laminating divided electrodes 74a and 74b, ZnO piezoelectric material 75, intermediate electrode 73, ZnO piezoelectric material 75' and divided electrodes 72a and 72b in this order, followed by removing a part of the silicon substrate under the cantilever by anisotropic etching.
Metal probe 77, provided on one end of the piezoelectric bimorph cantilever by adhering or the like, can detect tunnel current through a drawing electrode 76.
The cantilever can be moved independently in three dimensions by controlling voltages applied on four regions of piezoelectric material which comprises two regions sandwiched between upper divided electrodes 72a and 72b and intermediate electrode 73 and two regions sandwiched between lower divided electrodes 74a and 74b and intermediate electrode 73.
FIGS. 33 (a) to (c) are illustrations showing three dimensional motions of a prior art cantilever achieved by changing combinations of regions to which voltages are applied within four regions of piezoelectric material divided by pair of divided electrodes.
FIG. 33(a) shows the motion of a cantilever which can move probe 77 in the Y-direction shown in FIG. 31, when voltages with the same phase are applied so that four regions can contract simultaneously. FIG. 33(b) shows the motion of a cantilever which can move probe 77 in the X-direction shown in FIG. 31, when an upper and a lower region in the right side, stretch and an upper and a lower region in the left side, contract. FIG. 33(c) shows the motion of a cantilever which can move probe 77 in the Z-direction shown in FIG. 31 when a right and a left region in the upper side contract, and a right and a left region in the lower side, stretch.
FIGS. 34(a) to (d) schematically show a process for producing a prior art cantilever probe by employing a micromachining technique.
On both sides of a silicon substrate 1 with (100) crystal face, mask layer 79, which is used to etch the silicon substrate by anisotropic etching, is formed. An example of anisotropic etching is disclosed in "PROCEEDINGS OF THE IEEE", Vol. 70, No. 5, May 1982. Next, an opening is provided for anisotropic etching on the mask layer of the second surface, photolithographically (FIG. 34(a)). Photolithographical process is disclosed, for example, in "Integrated Electronics, " pp. 78-81, July 1983 by Corona Co. On the first surface of the silicon substrate, an electroconductive layer which becomes an electrode, is formed, followed by patterning photolithographically to form an electrode 74. 0n electrode 74, piezoelectric material 75 is formed, followed by patterning. Similarly, electrode 73, piezoelectric material 75 and electrode 72 are formed (FIG. 34(b)). A cantilever is formed by removing a part of the silicon substrate under a cantilever-like pattern from the second surface of silicon substrate 1 by anisotropic etching (FIG. 34(c)). On one end of the cantilever thus prepared, probe 77 is formed by adhering a metal piece such as Pt. Rh or W to prepare a cantilever probe (FIG. 34(d)).
By employing such a micromachining technique, it is possible to form a fine cantilever probe and also to form a multi-probe which is required to improve the speed of writing or reading-out of information in a recording-reproducing apparatus.
In such apparatus, however, in order to laminate films of electrode and piezoelectric material, it is necessary to properly control the thickness and the stress of each layer. That is because bending of a cantilever produced by etching a silicon substrate, occurs, depending on the film thickness and the stress of each layer.
The bending amount (.DELTA.) of the cantilever shown in FIG. 31 and FIG. 32 in the longitudinal direction is in proportion to the following formula: ##EQU1## Wherein t1, t2, t3, t4 and t5 respectively denote the thickness of electrode 74, piezoelectric material 75, electrode 73, piezoelectric material 75' and electrode 72; .sigma.1, .sigma.2, .sigma.3, .sigma.4 and .sigma.5 respectively denote the stresses of electrode 74, piezoelectric material 75, electrode 73, piezoelectric material 75' and electrode 72.
The bending of a cantilever occurs because of: (1) error of the thickness t1, t2, t3 and t4 of each thin layer, (2) film thickness direction distribution of the internal stress of the film (when t2=t4, .sigma.2 .noteq..sigma.4), (3) the kind of substrate material and the surface roughness in lamination, and (4) the change of thermal expansion coefficient of the substrate caused by lamination.
The bending of a cantilever by the above makes it difficult to provide an appropriate spatial relationship between the probe and the medium and makes it difficult to keep the distances necessary to detect tunnel current. It is a severe problem where a plurality of probes are used. If each cantilever is not set up properly at each appropriate location, the benefits of a cantilever probe produced by a micromachining technique, is lost.
Moreover, in the prior art, there is a disadvantage caused by conducting the etching of a silicon substrate from the reverse surface, twice. In the first anisotropic etching as well as in plasma etching, the thickness or the size of a silicon membrane is not uniform because of a different etching rate or different thickness of a silicon wafer. Accordingly, the size of a cantilever produced is not uniform. Therefore, the amount of displacement of the probe responsive to a driving voltage is not uniform. It is especially difficult to control the amount of displacement of a probe in forming a plurality of cantilevers on a substrate.
Furthermore, it is necessary to use a double-sided, polished substrate to improve the accuracy of two-sided alignment or etching, because anisotropic etching is conducted from the reverse surface of the substrate.